AlGaN/GaN based Heterostructure Field Effect Transistors (HFETs) show a tremendous promise as switching elements for power electronic applications. The key requirements for high-power switches include high breakdown voltage (VBR), minimum conduction and switching losses and the highest switching frequency to cope with the modern trends in power converter design and allow for monolithically integrated power converter technology. The most important physical device dimension which governs the breakdown voltage in AlGaN/GaN HFETs is the gate-drain spacing LGD where most of the voltage drops in the pinch-off device condition. The challenge of achieving high breakdown voltage VBR with a minimum on-resistance RON translates into an optimal field profiling in the gate-drain region so that it is able to sustain the highest possible voltage at the lowest LGD value.
An ideal switch will block infinite voltage when OFF and pass infinite current when ON with no voltage drop across it (or in other words the switch has zero resistance when ON and infinite resistance when OFF) and at the same time be capable of switching at a frequency of infinity. As is evident from its definition, an ideal switch performance can never be achieved by a practical power semiconductor switch. However, the aim of the power semiconductor industry have been to achieve a device which has as low a resistance as possible when in ON state called “ON-resistance” (RON) of the device for a given maximum voltage it can block in OFF state known as “breakdown voltage” (VBR) of the device. Keeping the VBR/RON ratio as high as possible, a maximum device current in ON state known as “Imax” is tried to be achieved from as small an area “A” as possible of the device. A smaller area of the device has a smaller associated capacitance and hence results in device which can switch at higher frequency “f”. As all the above mentioned power device parameters like VBR, RON, Imax, A, f are important, it becomes difficult to quantitatively compare two power devices in terms of their performance as a switch if all the power device parameters are considered. To overcome this problem and fairly but quantitatively compare two power devices, a power device figure merit “VBR/(RON.A)” is defined where in the device with the higher value of figure of merit can be considered a better device than a device with lower value of figure of merit.
In the past, reports on the breakdown voltage of as-fabricated AlGaN/GaN HFETs show that the VBR increases with LGD curve up to LGD≈10 μm, beyond which VBR saturates at around 400-450V. Single or multiple field plates (overlapping gate) have been implemented to increase the breakdown voltage. The HFETs with single field plate gate structure show a saturation breakdown voltage of 570V at LGD=13 μm. The HFET with multiple field plates demonstrates the breakdown voltage of 900V for a device with LGD=24 μm. The mechanism of the VBR increase in the field-plated devices is believed to be the electric field spike reduction at the drain side edge of the gate.
The present inventors have discovered that in HFET devices, either with or without the field plates, the breakdown voltage is limited by a surface flashover that occurs in the air regions adjacent to the gate-drain area, and is not due to the breakdown of the III-Nitride material itself. Suppression of this parasitic air breakdown by immersing the devices in a high dielectric strength liquid material like Flourinert® (3M Company, St. Paul, Minn.) results in linear VBR-LGD dependence reaching breakdown voltages as high as 1600 V at LGD=20 μm. Similar linear VBR-LGD curves with parasitic air-breakdown suppression have also been demonstrated on AlGaN/GaN HFETs with integrated slant field plates achieving 1900V at LGD=20 μm. The method of suppressing the surface flashover in the air by immersing in Fluorinert® clearly demonstrates the feasibility of achieving very high breakdown voltages in AlGaN/GaN HFETs. However, such method can hardly be considered as a practical way of fabricating devices for high-voltage power converters.
Hence, a need exists to find an alternative way to suppress the surface flashover without the need of immersing the devices in the liquid.